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https://issues.apache.org/jira/browse/FLINK-9422?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16489261#comment-16489261
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Hequn Cheng commented on FLINK-9422:
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Thanks for your explanation. :)
> Dedicated operator for UNION on streaming tables with time attributes
> ---------------------------------------------------------------------
>
> Key: FLINK-9422
> URL: https://issues.apache.org/jira/browse/FLINK-9422
> Project: Flink
> Issue Type: Improvement
> Components: Table API & SQL
> Reporter: Fabian Hueske
> Assignee: Ruidong Li
> Priority: Minor
>
> We can implement a dedicated operator for a {{UNION}} operator on tables with
> time attributes. Currently, {{UNION}} is translated into a {{UNION ALL}} and
> a subsequent {{GROUP BY}} on all attributes without aggregation functions.
> The state of the grouping operator is only clean up using state retention
> timers.
> The dedicated operator would leverage the monotonicity property of the time
> attribute and watermarks to automatically clean up its state.
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